Intermittent Tachometer

A

aarpcard

True Classic
So my '85 developed this problem recently.

For the last several days, when first starting the car, the tachometer will not register for about 5 seconds. Strangely the tach seemed to spring to life around the same time the delay relay for the seat belt buzzer switched off. . .

Today, the tach stopped working completely after about an hour of driving.

What's the best way to diagnose the issue here? I'm thinking it could be an issue with a ground in the dash/cluster (I've already cleaned all the grounds under the dash, but haven't touched the cluster yet. Or is it likely to be an issue with the sender from the ignition module . . . or something else completely.

When it works it is accurate, when it doesn't work, it is pegged at 0 RPM. What order would you guys suggest checking things in?

Thanks.
 
My '87 had the same issue. It is the coil in the tach. Impossible to fix, you have to replace the tach. Make sure you find one that works with the ignition module, '79 or later I think...
 
Joe F said:
My '87 had the same issue. It is the coil in the tach. Impossible to fix, you have to replace the tach. Make sure you find one that works with the ignition module, '79 or later I think...
Click to expand...

Good to know. I have a spare cluster from my 83 parts car. I'll swap the tach from it over if I have to - should be interchangeable, right?

I'll first verify that power, ground, and the signal from the ignition module are all good. None of the other gauges or lights have any issues, so I'm assuming power and ground for the cluster is good. Probably just gonna hook up the rpm send signal to an oscilloscope and see if anything looks strange.
 
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